1. Field of the Invention
This invention relates to laser scanning devices, such as those used in laser printers, and specifically to systems and methods for aligning laser scanning devices.
2. Related Art
FIG. 1 shows a conventional monochrome laser printer 100. Laser 102 is directed through mirror 104, which is typically a polygonal mirror, but in some implementations is a prism. Photoconductive drum 106 is charged as it rotates by corona wire 108. The laser 102 discharges a small spot on drum 106. Toner hopper 110 and developer roll 112 deposit charged toner on drum 106. The toner is charged with the same charge as the drum 106, so toner only adheres to the discharged spots produced by the laser 102. As media 114, such as a sheet of paper passes through, it is charged opposite to the drum 106 so that the toner is transferred to media 114. A fuser 116 fuses the toner to the media 114.
An alternative approach is to charge the toner with a charge opposite that of the drum. In which case, the toner would be attracted to the charge on the drum rather than repelled by the charge. In which case the laser would discharge the drum where no toner should be placed.
The laser, mirror and other optical components are collectively referred to as the “laser scanning unit” or “optical unit.” The laser scanning unit, along with the photoconductive drum, the fuser and other mechanical parts are collectively referred to as the “laser engine.” The electronics which drive the laser engine including the laser scanning unit is often referred to as the “engine controller.”
Operation of a color laser printer is similar to a monochrome printer, but the process is repeated for each color used. Conventional color laser printers use a four color printing model employing the primary colors of cyan, yellow, and magenta, along with black (“CYMK” color model). The earliest laser printers used a single laser which wrote the four colors on a single photoconductor drum in four sequential passes. This insured perfect alignment of the color planes because the same laser scanning unit is used to write each color.
A drawback with these sequential printers is that requiring the four individual passes can take up to four times the time to print a page over a single pass. Faster printing is achieved by using four laser scanning units to expose each of the four CYMK color planes in a single pass. In certain implementations these single-pass printers (also known as “inline printers”) include a complete printing unit, including a photoconductor drum, corresponding to each laser scanning unit.
Inline printers have an increased complexity with the alignment of the color planes. Improperly aligned color planes—for example due to misregistration, skew or mismatched size of color planes—degrade print quality and produce artifacts similar to a badly printed copy of color newspaper comics.
With inline printers, the position of each laser scanning unit affects the color plane alignment. For example, the distance between each of the laser scanning units and the drum may vary slightly, resulting in slightly different color plane sizes, which cannot be reconciled by proper registration. The resultant effect is that somewhere on the page color aberrations will occur.
FIG. 2 illustrates two laser scanning units at slightly different distances from their drums. Laser 202 sends pulses which are directed by mirror 204 (or prism) onto drum 206. The distance from mirror 204 to drum 206 is shown as x. The scan line produced on drum 206 has a length of γ. Laser 212 sends pulses which are directed by mirror 214 onto drum 216 (or drum 206 in an alternate implementation). The distance from mirror 214 to drum 216 is shown as x+δ. The resultant scan line from this laser scanning unit is shown as γ+ε. If laser 202 and 212 are modulated with the same dot clock or at the same frequency, triangles ABC and DEF are similar triangles. In this case, if δ is ¼% the length of x, the resultant error ε would be ¼% the length of γ, which is a mismatch of 6 dots at the extremes of the scan line on a standard letter sized page of 8½ inches wide at 600 dots per inch.
Known calibration techniques can be used to measure and correct the error of the printed scan line. One technique is to mechanically adjust the distance between the laser scanning unit and the drum. This often requires manual adjustment, or motor controlled adjustments for automatic calibration, which can be very expensive. Another method inserts “fake” additional dots or removes dots in a systematic way to compensate for the difference in scan lines in an attempt to hide the aberration throughout the printed page. The difficulty in this approach is that the deletions and insertions of dots may be visible due to the uniformity of the mismatched dots on each printed scan line.
Another solution is to increase or decrease the laser writing frequency to narrow or widen the printed scan line. The laser writing frequency is commonly implemented using frequency synthesis with phase locked loops (PLLs). While PLLs are a traditional way of synthesizing frequencies, but they are relatively expensive.
Some inline color printers share components of the laser scanning unit. For example, the four laser scanning units could share a single polygonal mirror, which could eliminate the alignment problem described above in FIG. 2. However, other laser printers have two mirrors where two laser scanning units share a mirror. In this case, there would still be an alignment problem.
FIG. 3 illustrates a typical electronic implementation of a CYMK laser printing apparatus using separate clock circuits using PLLs for each laser scanning unit. For each color, a laser is modulated by a control circuit which controls for each position whether a dot is written. For the purposes of nomenclature in this disclosure, each laser is associated with a color, and is referred to by that color. It does not imply that the laser actually produces that color. For example, the cyan laser described below labels the laser used to expose the cyan color plane on the photoconductive drum, not that the cyan laser generates a cyan colored beam. The control circuit is regulated by a dot clock which comprises a PLL circuit. For example, cyan laser 302 is controlled by control circuit 304 which uses dot clock 306 which contains PLL circuit 308. Yellow laser 312 is controlled by control circuit 314 which uses dot clock 316 which contains PLL circuit 322, and magenta laser 322 is controlled by control circuit 324 which uses dot clock 326 which contains PLL circuit 328. Finally, black laser 332 is controlled by control circuit 334 which uses dot clock 336 which contains PLL circuit 338. While the various control circuits can be implemented as a common control circuit, to account for the deviations in the distance from the laser scanning units to their respective drum(s) the circuitry for the various dot clocks are essentially distinct circuits and the frequencies are individually tunable.
FIG. 4 shows another alignment problem in laser printers. Error in the synchronization of the dot clock, rotating mirror and advancement of the drum from one line to another may result in the location of the first dot of each line varying based on when during a dot clock cycle a new line begins. As shown, clock signal 402 is shown with a corresponding row of dots 412. When the second row of dots is printed, the clock is askew by a ⅓ of a clock cycle as shown by clock signal 404 relative to the start of printing of the row. As a result, the corresponding row of dots 414 is indented by ⅓ of a dot. Similarly when the third row is printed, clock signal 406 is askew by a ⅓ of a clock cycle from clock signal 404 relative to the start of printing of the row. Row of dots 416 is indented by another ⅓ of a dot. In this example, clock signal 408 is further askew by another ⅓ of a clock cycle, putting it back in sync with clock signal 402. Row of dots 418 is now aligned with row 412. The dotted line shows the skew effect. When this alignment causes a subtle aberration it may be ignored. However, if each color plane operates with a dot clock at a slightly different frequency, the skew per row will may be different for each color magnifying the aberration.
The rotation of the mirror that directs the beam from a laser can cause further distortion. The mirror generally rotates at constant angular velocity. Suppose for notational sake, the mirror spins at an angular velocity of ω. If a laser is on for a time interval of Δt, the mirror has an angular displacement of θ=ωΔt. When the beam starts out perpendicular to the page, the spot created by the laser that is on for Δt is smaller than the spot that is created by the laser when the initial angular displacement of the beam is larger. FIG. 5A shows the spot size δ when the initial angular displacement is 0. In contrast FIG. 5B shows the spot size δ when the initial angular displacement is φ>0. Spot size δ′ can be several times greater than δ, which if uncheck can cause significant distortion. For example, FIG. 5C shows a plurality of dots as distorted by the constant angular velocity of the mirror. Dot 512 in the center would be the proper size while dots 514 closer to the edge of the printable region would be elongated. If the dot frequency were shorter then dots near the edge could be shortened to the proper size, but the dots near the center would be compressed.
One solution is to vary the angular velocity of the mirror to obtain a constant linear velocity across the drum. Such mechanical control of the mirror is complicated and expensive to achieve. Conventional systems use a system of optics including aspherical lenses to approximate a constant linear velocity to produce dots of consistent size, by making the optical path longer so that the amount of error is reduced to acceptable levels. In order to improve performance and reduce the size of the laser scanning unit, more complex optics are employed including the addition of mirrors, diffractive optics and light pipes. Including such optics within the correct tolerances can also be expensive. Including such optics within the correct tolerances can also be expensive.
Accordingly, it is desirable to control the width of the four color planes, the alignment of the dots between rows and the compensation for constant angular velocity of the mirror in an inline laser color printer.